Systematic characterisation of site-specific proline hydroxylation using hydrophilic interaction chromatography and mass spectrometry.

We have developed a robust workflow to identify proline hydroxylation sites in proteins, combining hydrophilic interaction chromatography (HILIC) enrichment and high-resolution nano-liquid chromatography-mass spectrometry (LC-MS) with refining and filtering parameters during data analysis. Using this approach, we have combined data from cell lines treated with either the prolyl hydroxylase (PHD) inhibitor, Roxadustat (FG-4592), or with the proteasome inhibitor MG-132, or with a DMSO control, to identify a total of 4993 and 3247 proline hydroxylation sites, respectively, in HEK293 and RCC4 cells. Of these, 1954 (HEK293) and 1253 (RCC4) high-confidence non-collagen sites were inhibited by FG-4592. Hydroxylated peptides showed consistent characteristics across both datasets, including enrichment in more hydrophilic HILIC fractions and distinct charge and mass distributions compared to unmodified or oxidised peptides. The intensity of the diagnostic hydroxyproline immonium ion varied with MS collision energy, peptide concentration, and adjacent amino acid sequence. Using synthetic peptides, we demonstrate that combining LC retention time with optimised MS parameters enables reliable site identification, even with multiple proline residues present. Proteins with FG-4592-inhibited hydroxylation sites were enriched for roles in RNA metabolism, mRNA splicing, and cell cycle regulation, including the phosphatase 1 regulatory subunit Repo-Man (CDCA2).